1. Field of the Invention
This invention relates to a deceleration sensing type proportioning valve which is capable of appositely distributing front and rear wheel braking forces at the time of braking a vehicle.
2. Description of the Prior Art
It is known that, generally in order to obtain an effective wheel braking force by having a suitable slip ratio between wheels and a road surface, braking forces on rear wheels must be arranged to be lower than the front wheels. This is because the front wheels are pressed against the road surface by a greater force than the rear wheels and, because of variation in the directionality of the force of inertia, the ideal braking force distribution ratio of the front wheels to rear wheels becomes as represented by the characteristic curves a, b and c shown in FIG. 1 of the accompanying drawings. Increase of loadage W mainly results in increase in the force of pushing the rear wheels against the road surface. Therefore, the above stated characteristic curve varies with the increase of loadage in such a way as a.fwdarw.b.fwdarw.c as shown in FIG. 1 and thus gradually approaches a front wheel-to-rear wheel braking force ratio Fr/Ff=1/1.
It is desirous to bring an actual front wheel-to-rear wheel braking force ratio closer to the ideal braking force distribution ratio. In view of this, therefore, a control valve such as a proportioning valve or the like has heretofore been inserted in a hydraulic line connecting a hydraulic source (a master cylinder) to a rear wheel braking device.
In the conventional valve of this type, hydraulic oil pressure to be transferred is arranged to begin to gradually increase when it reaches a predetermined value which is set as a turning point. However, since this turning point is fixed, in the case of a vehicle such as a truck that has a great variation in live load, the actual control characteristic curve has been deviating to a great degree from the ideal curve.
This problem may be solved by adding a load responsive mechanism which detects relative displacement of a vehicle body and an axle resulting from a change in the load and which changes the above stated turning point of the hydraulic pressure in response to the relative displacement. However, such arrangement results in a disadvantage such as a complex structure, increase of parts, a rise of cost, etc.
Further, in connection with the solution of the above stated problem, U.S. Pat. No. 3,825,303 discloses a deceleration sensing type proportioning valve. In this proportioning valve, there is provided a plunger which confronts an adjustment piston within a cylinder; a control spring which determines a turning point hydraulic pressure value is provided between the plunger and the adjustment piston; and the turning point hydraulic pressure is arranged to be increased by compressing the control spring approximately in proportion to the increase of the value of hydraulic pressure shut in within a hydraulic adjustment oil chamber.